I have a boxer/lab mix puppy. He is 13 weeks and 20 pounds. He had parvo as a puppy. I was wondering if there was a way to know how big he will be.

I would guess he will hit the range for Labs and Boxers. As near as I can tell, dogs get as big as they want. There are a number of rules, but none of them seem to work.

In other areas, there are things that work. I have many of them in a sticky at https://www.askmehelpdesk.com/dogs/information-articles-our-dogs-expert-labman-53153.html#post251802